1. Field of the Invention
The present invention relates to selective optical filtration in the ultraviolet region of the spectrum. More particularly, the invention relates to the incorporation of organic dyes, which block unwanted ultraviolet (UV) wavelengths, in substrates that offer good transparency in the UV region of the electromagnetic spectrum to wavelengths as short as 200 nm.
2. Description of Related Art
Optical filters are passive components whose basic function is to define or improve the performance of optical systems. Applications of optical filters generally can be as diverse as anti-glare computer screens, laser devices such as ophthalmic surgical lasers, sighting devices, etc. Many applications and instruments exist where optical filters are used to tune the optical behavior of light in the ultraviolet range (typically wavelengths shorter than 400 nm). Some applications include water purification, blood chemistry analysis, and the chemical evaluation of foods, pollutants, gases, etc. More specifically, optical filters can be used in many biomedical analysis systems, e.g., to detect the presence of a specific marker (such as an enzyme) in a blood or tissue sample. The marker is customarily identified by fluorescence upon exposure of the sample to a detection wavelength, using a filter that specifically does not autofluoresce.
Particular applications for solar blind filters include inspection of high voltage transmission lines for corona and partial discharges, detection of forest fires, detection of fires in industrial installation, missile plume detection, monitoring Cherenkov light, monitoring lightning events during thunderstorms, detecting ultraviolet laser sources, such as excimer lasers or frequency quadrupled Nd:YAG lasers used as LIDAR sources, and ultraviolet telescope detectors for space platforms. The detection of ultraviolet (UV) light during daylight conditions is an important problem for both commercial and military applications. Many important phenomena produce UV radiation, including fires and flames, rocket and jet engine exhausts, electrical corona and electrical discharges on high tension wires, lightning, and the plasma surrounding an object that is entering the earth's atmosphere at a high velocity.
For example, corona discharges in air emit light mainly in the 230–405 nm range. The most intense emissions are the 317, 337 and 357 nm lines. These emission lines are, however, very weak relative to daytime background solar radiation. It is, therefore, not possible to use these emission lines to image corona during the daytime, even when narrow bandpass filters and applying background subtraction method. Although the corona emission in the solar blind range is much weaker than the UVb and UVa lines, by using a truly solar blind filter that almost totally blocks the background radiation outside the bandpass, UV images of high contrast can be obtained.
As another example, strike and interceptor aircraft operating in a hostile environment can be detected positively by early warning systems detecting UV from the missile plume, but must be sensitive in the region of the missile plumes' UV radiation, from about 230–280 nanometers, to reduce false alarms. Further, such detectors should have a large area, wide field of view.
Photons of ultraviolet radiation are more energetic than photons of visible light and are therefore theoretically easier to detect. Although the sun always emits a large quantity of radiation at a wavelength below about 290 nm to less than 285 nm, such radiation is usually efficiently absorbed by the atmosphere, specifically by ozone in the stratosphere. Thus, as shown in FIG. 1, the solar radiation reaching the earth surface drops rapidly as the wavelength decreases. Accordingly, one advantage of detection in the low wavelength range of the UV is the relative absence of background radiation; weak UV signals in this band can be imaged with a relatively high signal to background ratio, especially at night.
However, due to the extremely high actinic flux of solar radiation, it is still difficult to design a very sensitive optical system that can be used in broad daylight to detect very low levels of UV radiation. The spectral distribution of radiation from the sun is similar to that of a 6,000 degree black-body radiator, and the detection of an ultraviolet emission source, such as a fire or a rocket plume, during the day is complicated even by the modest amount of UV light emitted by the sun that is not fully absorbed by the earth's atmosphere. A solar blind filter may typically be defined as a filter that blocks both visible and the longer UV wavelengths (UVa and UTb) and only transmits UV wavelengths below about 285 nm (sometimes referred to as UVc), as shown in FIG. 1. Solar blind signal-to-noise ratios are maximized when the actinic flux of solar radiation is completely excluded from the sensor. As the wavelength of light becomes shorter in the UV range, prior optical filters suffer from many disadvantages, such as poor optical performance, limited physical longevity, high autofluorescence, poor imaging quality of the transmitted radiation, and transmitted wavelength instability.
One approach has been to use absorption type materials using large single crystals. For example, U.S. Pat. No. 4,731,881 discloses a chemical filter comprising a single crystal nickel sulfate hexahydrate crystal. However, this material, as well as filters using rare earth ions as crystalline hosts, are very high in cost because of the need to grow single crystals. Moreover, they typically have poor thermal and moisture stability.
Some filters incorporate organic dyes in polymeric substrates, such as polyvinyl alcohol. For example, U.S. Pat. No. 4,731,881 also describes the use of an organic dye, referred to as Cation X, contained in a polyvinyl alcohol film, to provide the desired UV bandpass characteristics. U.S. Pat. No. 6,126,869 discloses a solar blind optical filter that includes a carrier material and the salt of a dithioic acid of the formula RCS2−X+, wherein R is an organic substituent that does not absorb ultraviolet light at wavelengths between 260 nanometers and 300 nanometers, and X is a counterion. However, the polyvinyl alcohol substrate in which the organic dye is incorporated in these filters is also susceptible to changes in humidity and to environmental degradation.
Still other means include glass substrates, but glass materials often result in broader absorptions than desired, and may tend to fluoresce. Thus, they are not usually desirable as filter materials for optical reasons. Further, they require fabrication at high temperatures to solvate materials to be incorporated in the glass. Specifically, even if UV-transparent glass were used, it would not be possible to dissolve the organic dyes desired into UV transparent glass substrate, because the organic dyes would be unstable at the high processing temperatures required.
U.S. Pat. No. 6,126,869 also discloses that an organic dye may be absorbed onto silica nanospheres or immobilized in a gel or glass matrix. However, these substrates would either produce an unacceptable degree of scattering or would exhibit some of the same problems experienced with glass discussed above.
Layered dielectric or rugate filters can also be used as wavelength-selective filters in the UV region of the spectrum, but they have a strong angular dependence to the filtering action and are also expensive to produce. Therefore, they would not be acceptable for tracking UV-emitting sources over a large area or wide angle of view.